1. Field of the Invention
The present invention relates to a bushing and, more specifically, to a bushing provided with internal shields suitable for reducing electric field concentration on the surface of the bushing.
2. Description of the Related Art
A conventional bushing is provided with a cylindrical shield coaxial with a central conductor and mounted inside an insulating tube, and an external shield ring mounted outside the insulating tube to control an external electric field.
A bushing disclosed in Japanese Patent Laid-open No. 58-163111 has a central conductor, a capacitor tube or a shield electrode for potential adjustment mounted so as to surround the central conductor, an insulating tube, a short insulating tube connected to the inner surface of the insulating tube, and an electrode for electric field relief mounted near the joint of the short insulating tube and the insulating tube. A bushing disclosed in Japanese Patent Laid-open No. 60-86709 has a central conductor, a first annular shield kept at a ground potential and mounted coaxially with the central conductor, and a plurality of annular shields supported in a stack by an impedance support member with the annular shield at an end of the stack mounted inside the first annular shield and kept at a potential other than the ground.
In the related art bushing, the coaxial cylindrical shield has a great height along the axis to control an electric field. Therefore, as is obvious from an equipotential distribution diagram shown in FIG. 11, all the potentials are raised axially along a cylindrical shield 110, potential is concentrated on a space near an upper part of the coaxial cylindrical shield 110, and the potentials are distributed in an outer space. Consequently, the electric field is concentrated on a part of the surface of the insulating tube 101 near the upper end of the coaxial cylindrical shield 110, which causes corona discharge under wet condition and deteriorates the antipollution ability. In particular, when a composite insulating tube formed by coating the surface of an insulating tube with an organic material, such as silicone rubber is employed, corona discharge in a wet state deteriorates the surface of the insulating tube, reduces reliability in insulation and the lifetime of the bushing may be shortened.
The bushing disclosed in Japanese Patent Laid-open No. 58-163111 has stacked internal shields and therefore has a problem in reliability in its insulating performance because the stacked internal shields may possibly be shifted or moved by earthquakes or mechanical vibrations of gas-insulated switchgear and the like. An internal shield internally with an electric field relieving shield, a connector on and a triple junction, cannot be achieved.
The plurality of shields of the bushing disclosed in Japanese Patent Laid-open No. 60-86709 cannot perfectly be gas-insulated because some parts of the shields are connected to the conductor by an impedance member. The provision of potential by impedance is likely to change with time. Since the impedance member is placed at the end of the shield where the intensity of the electric field is high, the dielectric strength is lower than that of the insulating member and reliability in insulating performance is not satisfactory.